With respect to methods and granulators for granulating urea, urea/ammonium sulfate, etc., many proposals have been made. For example, the inventors of the present invention have proposed, as granulation methods and granulators in which a combination of a fluidized bed and a spraying (injection) bed is used, for urea, a method for working (processing) particles, as disclosed in JP-B-4-63729 ("JP-B" means examined Japanese patent publication), a granulation method and a granulator that are improvements of the method disclosed in the patent publication above, and further developed a method for producing urea/ammonium sulfate fertilizer granules. On the other hand, an improved method for granulating urea, in which a fluidized bed is used, is disclosed in JP-B-56-47181, and a method for producing granules each made up of a core and a coating layer is disclosed in JP-B-60-13735.
Out of such conventional or preceding granulation methods, a representative method will be described below with reference to FIG. 18.
In FIG. 18, at the start-up, seed particles of urea are fed as nuclei to a granulator 1 (the A-type granulator described later), through a line 41 from a line 40, which is a feed port of the line. In the granulator 1, an aqueous urea solution containing 90% by weight or more, preferably 95% by weight or more, of urea is sprayed as liquid droplets, having a diameter of 150 to 600 .mu.m, to the nuclei at a prescribed spray angle chosen from 30 to 80 degrees, from nozzles 6, 7, and 8. Further, molten urea 17, having a concentration of 90% by weight or more, preferably 95% by weight or more, fed from a urea synthesis plant or the like (not shown), wherein the temperature of the molten urea is adjusted to 125 to 145.degree. C., is fed from a line 31 to a mixing tank 21, and then it is fed through a line 36, a pump 22, and a line 37, to the nozzles 6, 7, and 8.
Upon spraying the above seed particles of urea fed from the line 41 with the aqueous urea solution in the granulator 1, the seed particles grow and are stirred up to a space 60 by jetting currents from air feed pipes 3, 4, and 5, branched from a lower air feed pipe 2, led from a line 24, which is a lower feed port, and the particles are permitted to drop as a grown granular urea 70, in a lower space 11 from a state 10 in which urea particles are stirred up. On the other hand, fluidization air is fed from a line 23, which is an upper feed port, so that the grown granular urea 70 on a bottom floor 9, having multiple opening holes perpendicular to the bottom, are kept in a fluidized state in the space 11 to the extent of a level 12, and the granular urea that is growing is fluidized, to fill all the space 11 over the nozzles 6, 7, and 8.
The above movements are repeated, and the granular urea thus formed is finally discharged from a line 25, which is a discharge port.
The proportion of those of a nominal product size among the granular urea discharged from the line 25 of the granulator 1 (hereinbelow, this proportion is referred to as a content of the nominal product size at the granulator outlet.), is generally 75 to 80%, as shown in Comparative Example 1 described below, and the granular urea is sifted through a sieve 13, to be separated into a standard (on-specification) product and a nonstandard (off-specification) product, with respect to the desired content of the nominal product size in a product. The standard product is passed through a line 26, to be stored as the product 14. On the other hand, to keep the number of nuclei in the granulator 1 constant, in view of stable continuation of production of the product, the product having a particle diameter greater than the specified particle diameter, and part of the standard product, are passed through a line 27 into a crusher 15, wherein they are crushed; the product having a particle diameter smaller than the specified particle diameter is passed through a line 28 and is added to line 29; and the mixture is passed through a line 30 and the line 41 to the inlet of the granulator 1, to be recycled as nuclei for the granulation.
Further to this discussion, it is well known that, in this recycling, when urea granules are crushed by using the crusher 15 to form smaller particles, a crushed product having a broad distribution of particle diameter is formed containing a large amount of powder, and the energy consumption for the crushing is large. As a result, when such a crushed product is recycled as nuclei to the inlet of the granulator 1, the occurrence of a large amount of dust in a fluidized state cannot be avoided.
Further, the recycling of a crushed product is not preferable in view of the quality of the product. The product recycled as nuclei to the line 41 of the inlet of the granulator 1 is crushed product, and therefore it is not spherical. Such crushed pieces are coated in the granulator 1, to take rounded shapes, and they are discharged from the granulator 1 with their shapes remaining odd-shaped that can be evaluated by the method described in detail later. As a result, with respect to the size, the resulting product comes up to a standard product, but with respect to the shape, the resulting product contains odd-shaped granules and is quite lowered in product value.
As described above, when the product crushed in the crusher 15 is recycled as nuclei to the inlet of the granulator, a large amount of dust is generated in the granulator, the yield of the product is poor, and the product contains odd-shaped granules.
In the method for producing urea granules, as described in JP-B-56-47181, the granulator shown in FIG. 2 therein is of a fluidized bed type, and it can be seen that product crushed in a crusher in the production process is recycled as nuclei to the granulator. Accordingly, even this method cannot solve the problems that dust is generated, the yield of the product is poor, and odd-shaped granules are included.